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Let's bring back the salmon, sea birds and G6 mackerel! THE HYPOTHESIS ON OVERGRAZING AND PREDATION

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Let's bring back the salmon, sea birds and G6 mackerel! THE HYPOTHESIS ON OVERGRAZING AND PREDATION

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This is a summary of the 'The hypothesis on overgrazing and predation' written in 2018. If you want to a deeper understanding of the serious situation which has arisen as a consequence of unsustainably large pelagic fish stocks overgrazing the NE Atlantic, download the hypothesis paper and read it carefully to understand. SUMMARY The development of 'The hypothesis on overgrazing and predation' started around 2005. It has been a dynamic process where new observations and data have been included along the path. Looking through the talks on Researchgate.com that I have given on the subject since 2008 this is evident. The development of the hypothesis will continue forwards. Today it reads: a) Over a long time, a systematic underestimation of the pelagic fish stocks in the Norwegian Sea has occurred, especially in mackerel and herring but probably also blue whiting. This has resulted in systematically too low quota advice and too low fishing levels. The stock growth in the pelagic fish stocks in the Norwegian Sea and neighboring areas has consequently been larger than we have been aware of for a long time. b) Too large stocks of pelagic fish have led to a strong overgrazing of the zooplankton and juveniles/small fish in the Norwegian Sea and adjacent coastal and marine areas. The recommended quotas have not been ecologically sustainable. c) Due to the mackerel's strong population growth, its opportunistic character and high migratory potential, it has increased its spawning and grazing areas dramatically. d) In parallel with the decreasing plankton resources, the mackerel has compensated for the reduction in the plankton food source by changing its feeding habits to eat more juveiles/small fish like larvae and 0-group of Norwegian spring spawning herring (NSSH), capelin at Jan Mayen Island, sprats in the Norwegian fjords, sand eel and salmon post smolt in the early sea phase. e) The strong grazing pressure from the mackerel stock has consequently a serious negative impact on many species today, both through direct and indirect competition for food and through predation. f) The strong down-grazing of zooplankton that grazes the plant plankton has led to a strong reduction in the general grazing on the plant plankton. Therefore, today much of the plant plankton dies off and sinks to the bottom rather than being eaten by the zooplankton. g) Because of the energy from the plant plankton lost to the bottom we see that the total productivity of the Norwegian Sea ecosystem is greatly reduced and more and more stocks in the upper part of the ecosystem are deteriorating (Figure 1). h) Examples of this deterioration are collapses in many zooplankton and small fish-dependent sea bird stocks, collapse in European salmon stocks, collapse in the individual growth of the mackerel stock and collapses in local herring, sand eel and sprats stocks on the Norwegian coast. We also for instance observe a strong reduction in the blubber thickness of the minke whale, a change from Norwegian Sea feeding to Barents Sea feeding for fin whales in summer and changes in the sperm whale's feeding habits outside Vesterålen in northern Norway during summer. i) The lowered ecosystem productivity also give a lowered output for the fisheries, for instance reflected in lack of large and high quality mackerel and low recruitment in the Norwegian spring spawning herring stock. j) We have already seen many negative effects of the pelagic fish stocks overgrazing the Norwegian Sea and must be prepared for more and more serious ecologic and economic consequences in the future unless the current trend of overgrazing by mackerel is actively reversed.
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Let’s bring back the salmon, sea birds and G6 mackerel!
THE HYPOTHESIS ON OVERGRAZING AND PREDATION
Jens Christian Holst
jens@ecosystembased.com
Version 26th June 2018
Contents
Foreword by Tony Andrews former Director of the Atlantic Salmon Trust (AST)………… Page 2
Chapter 1: Is overgrazing and predation by mackerel to blame for declining
European salmon and sea bird stocks today?............................................................... Page 4
Chapter 2: Empiric basis for claiming that the northeast Atlantic mackerel stock is
grossly underestimated…………………………………………………………………………………………..... Page 19
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Foreword
The Hypothesis on Overgrazing and Predation by Mackerel in the northeast Atlantic.
The JCH hypothesis claims that unprecedented numbers (literally billions) of mackerel are competing
with European salmon post smolts (approx. 10-15 million) for food and predating on them as
opportunities arise. The collapse in numbers of salmon (to about 5%) returning to their native rivers
points to a massive mortality at sea. The coincidence of the explosion in timing, density and range of
mackerel numbers with the sharp decline in returning salmon provides a possible and feasible
explanation. Our knowledge of salmon migration routes and where they overlap with massive mackerel
shoals leads to an assumption that overgrazing by billions of mackerel could leave little food for young
salmon.
At the same time both mackerel and salmon post smolts show signs of starvation and severe loss of
condition. With faster growing mackerel using the same sea space as salmon smolts throughout their
migration, opportunities for predation by mackerel are frequent
I asked Dr Holst if he would allow me to write this foreword to his hypothesis from the viewpoint of
someone who has worked closely with scientists, managers, anglers and people who depend on wild
salmon for their livelihood or lifestyle. I am not a scientist myself. I am however an experienced salmon
fishery manager and angler. My time working with the AST introduced me to the tumultuous world of
salmon politics, dominated by traditional netting (now largely gone), the salmon aquaculture industry
and the salmon angling tourism industry. Before I continue, I must explain that Dr Holst’s sole aim is for
his hypothesis to be tested.
Dr Holst’s calculations on current abundance of mackerel in the northeast Atlantic region challenge
official ICES estimates. Basing his thinking on official figures, and on his own observations and those of
fishing vessel crews in the region, he believes that mackerel numbers are being severely underestimated,
perhaps by a factor of six or more, and that this has led to dangerously low mackerel fishing quotas over
years.
Mackerel are now more numerous than ever before according to recent reports quoted in Dr Holst’s
paper. Their range now extends into areas such as the southern tip and east coast of Greenland, around
Iceland, Jan Mayen Island and Spitzbergen, where they have never been recorded before.
Impacts on other species. Resulting overgrazing of zoo plankton species has impacted on other pelagic
fish species. Certain species of plankton and small fish-eating birds, such as kittiwakes and puffins, have
also been negatively affected. Dr Holst also surmises that the explosion in mackerel abundance in
northern areas has led to the large fish-eating gannet showing strong population growth and establishing
colonies in new areas, such as Bear Island, from where they can now predate on mackerel.
I first met Dr Jens Christian Holst in the company of the AST’s Research Director, Dr Richard Shelton, at
the 2008 NASCO conference in Asturias. The SALSEA-MERGE project, for which Dr Holst became co-
ordinator, was about to become the first multinational effort under the NASCO banner to explore the
lives of salmon at sea. Dr Shelton and Dr Holst had previously worked together on sampling salmon post
smolts at sea. They were jointly responsible for designing and testing an open-ended trawl that enabled
photos to be taken of salmon as they passed through an open trawl. Those early sampling cruises
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provided evidence that mackerel and post-smolt salmon are included in species overlaps during their
northern, late spring migration west of the British Isles.
Dr Holst’s experience of studying interactions between pelagic fish species in their marine environment
is the basis of a practical approach to studying the lives of salmon at sea. By combining data coming from
marine research vessels with the experiences of sports and professional fishermen he adopts an
integrated scientific methodology. That ecosystem-based approach has become the baseline for his
hypothesis that there is now a serious imbalance among pelagic fish stocks, caused by the dominance of
mackerel overgrazing and predating on other juvenile fish species, such as salmon post smolts.
An ecosystem-based approach demands taking into account all factors that could possibly influence
recruitment, growth and survival of different species. Sea temperature and atmospheric pressure for
example are critical influences acting as ‘conductors in the ecological orchestra’ in which all pelagic fish
play varied roles at different stages in their life histories. The role of commercial fishing to manipulate
fish stocks to achieve a more balanced food web is also of crucial importance. This mechanism is
routinely used, for example in livestock production, deer management and trout lake management
If the JCH Hypothesis leads to the conclusion that huge shoals of mackerel are severely damaging the
pelagic food web in areas of the northeast Atlantic Ocean it will become necessary to reduce their
numbers through an internationally agreed and closely monitored thinning fishery. How that might be
achieved is the subject of a discussion to be held later, if and only if the hypothesis is shown to be valid.
TA
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Chapter 1
Background
Wild salmon stocks from in particular the European southern NEAC salmon area are dwindling
and today for instance most Irish and Northern Irish rivers are closed for ‘normal’ fishing.
Hypotheses on the marine factors underlying the salmon collapse are many: marine climate, sea
lice from salmon farms, inbreeding of escapees in wild salmon stocks, disease and bycatch in
pelagic fisheries.
The 2017 ICES Working Group on North Atlantic Salmon, ICES WGNAS, is clear in its conclusions:
The continued low abundance of salmon stocks across North America, de-spite significant
fishery reductions, strengthens the conclusions that factors acting on survival in the first and
second years at sea, at both local and broad ocean scales are constraining abundance of Atlantic
salmon’.
In parallel with the collapsing European salmon stocks a collapse has been observed in western
European sea birds eating plankton and small fish, like the kittiwake and puffin. The salmon and
these types of sea bird have in common that they are direct and indirect competitors with
mackerel for food.
We have also seen a degrading of the quality of the mackerel fished in the northeast Atlantic at
least over the last ten years, for instance with the so-called G6 quality more or less disappearing
from the catches. G6 is mackerel above 600 grammes that is highly regarded and well paid in
the Japanese market. In general, there are more and more negative signals coming out of the
markets about the degrading quality of the northeast Atlantic mackerel leading to more and
more severe consequences for the fishing industry.
As a marine fisheries scientist, I have worked closely on the marine ecology of salmon and the
factors affecting marine survival of Atlantic salmon since 1991. Based on my ecosystem-based
research in the NE Atlantic, I have developed the hypothesis that overgrazing and predation are
major factors behind amongst other dwindling salmon and sea bird stocks, and the
deteriorating quality of mackerel fished in western Europe.
Based on what I consider to be strong empiric evidence, the NE Atlantic mackerel stock has
grown totally out of proportion due to gross underestimation, leading to overly cautious fishing
quotas and underfishing as a consequence. Because of this very large mackerel stock, the food
resources of whales, seals, sea birds, salmon, other pelagic fishes and the mackerel itself are
now heavily overgrazed. Today, a 7-year-old mackerel weighs half of its weight of 10 years ago
a clear sign of the overgrazing and lack of food. This is only one of many signs of an
ecosystem totally outside its ‘natural range’.
This lack of food has also lead to starvation and very slow growth of young salmon at sea, the
salmon postsmolt. Postsmolts are now more vulnerable to predation and other natural
mortality than before the mackerel explosion.
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This document presents the hypothesis on overgrazing and predation with a focus on the effects
on western European salmon and sea bird stocks, but also mentioning other ecologic and
economic effects. Because underestimation of the NE Atlantic mackerel stock forms an
important basis for the hypothesis, I present data and argue in chapter 2 of this document that
ICES at present dramatically underestimate the NE Atlantic mackerel stock.
The hypothesis on overgrazing and predation
The development of the hypothesis on overgrazing and predation started around 2005. It has
been a dynamic process where new observations and data have been included along the path.
Looking through the talks on Researchgate.com that I have given on the subject since 2008 this
is evident. The development of the hypothesis will continue forwards. Today it reads:
a) Over a long time, a systematic underestimation of the pelagic fish stocks in the Norwegian
Sea has occurred, especially in mackerel and herring but probably also blue whiting. This has
resulted in systematically too low quota advice and too low fishing levels. The stock growth in
the pelagic fish stocks in the Norwegian Sea and neighboring areas has consequently been
larger than we have been aware of for a long time.
b) Too large stocks of pelagic fish have led to a strong overgrazing of the zooplankton and
juveniles/small fish in the Norwegian Sea and adjacent coastal and marine areas. The
recommended quotas have not been ecologically sustainable.
c) Due to the mackerel's strong population growth, its opportunistic character and high
migratory potential, it has increased its spawning and grazing areas dramatically.
d) In parallel with the decreasing plankton resources, the mackerel has compensated for the
reduction in the plankton food source by changing its feeding habits to eat more juveiles/small
fish like larvae and 0-group of Norwegian spring spawning herring (NSSH), capelin at Jan Mayen
Island, sprats in the Norwegian fjords, sand eel and salmon post smolt in the early sea phase.
e) The strong grazing pressure from the mackerel stock has consequently a serious negative
impact on many species today, both through direct and indirect competition for food and
through predation.
f) The strong down-grazing of zooplankton that grazes the plant plankton has led to a strong
reduction in the general grazing on the plant plankton. Therefore, today much of the plant
plankton dies off and sinks to the bottom rather than being eaten by the zooplankton.
g) Because of the energy from the plant plankton lost to the bottom we see that the total
productivity of the Norwegian Sea ecosystem is greatly reduced and more and more stocks in
the upper part of the ecosystem are deteriorating (Figure 1).
h) Examples of this deterioration are collapses in many zooplankton and small fish-dependent
sea bird stocks, collapse in European salmon stocks, collapse in the individual growth of the
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mackerel stock and collapses in local herring, sand eel and sprats stocks on the Norwegian
coast. We also for instance observe a strong reduction in the blubber thickness of the minke
whale, a change from Norwegian Sea feeding to Barents Sea feeding for fin whales in summer
and changes in the sperm whale's feeding habits outside Vesterålen in northern Norway during
summer.
i) The lowered ecosystem productivity also give a lowered output for the fisheries, for instance
reflected in lack of large and high quality mackerel and low recruitment in the Norwegian spring
spawning herring stock.
j) We have already seen many negative effects of the pelagic fish stocks overgrazing the
Norwegian Sea and must be prepared for more and more serious ecologic and economic
consequences in the future unless the current trend of overgrazing by mackerel is actively
reversed.
Figure 1. Upper panel: A ‘normal’ pyramid shaped oceanic ecosystem. The green arrows indicate
the normal flow of energy from the plant plankton to the zooplankton and upper part of the
ecosystem.
Lower panel: A ‘top-heavy’ ecosystem where the pelagic fish grazing capacity is too large and
has overgrazed the zooplankton. This has led to a severely lowered flow of energy from the
plants to the upper parts of the ecosystem with grave consequences to the total ecosystem
productivity, many species and the harvesting potential of the ecosystem.
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Mackerel, a voracious hunters
A 30 cm long mackerel will eat at least a 12.5 cm mackerel meaning a mackerel can eat a fish at
least 40% its own length (Figure 2). This again means a mackerel at 50 cm can eat a 20 cm
postsmolt salmon. In other words, mackerel today can prey efficiently on postsmolt salmon
during much of the postsmolts first summer at sea.
Figure 2: 30 cm mackerel with 12.5 cm mackerel in its stomach. Photo courtesy of Ian Kinsey.
Traditionally, the main spawning grounds of the NE Atlantic mackerel stock were in the North
Sea and west of the British Isles. In parallel with the strong stock growth starting around 2003,
mackerel spawning areas swelled, particularly in the western areas and northwards towards
northern Norway.
The widening of mackerel egg distributions from April-May 1992 (Figure 3) to 2016 (Figure 4)
demonstrate the increase of the mackerel spawning stock distribution west of the British Isles.
In figure 5 and 6 the observed expansion of the mackerel summer feeding areas from 2007 to
2017 is shown. Both the expansion of the spawning and feeding areas demonstrate the
enormous growth of the mackerel stock. From 2008 onwards, mackerel have also spawned in
the Norwegian Sea and in Norwegian fjords, as far north as northern Norway (Figure 7).
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Figure 3: Distribution of mackerel egg, proxy for spawning mackerel, during the period of the
Irish smolt run in 1992. Left 13 April 5 May, right 16 May-13 June.
Figure 4: Distribution of mackerel egg, proxy for spawning mackerel, during period of the Irish
smolt run in 2016. Left 9-30 April, right 1-30 May. Note that the survey does not find the
northern zero line of eggs which correspond well with the observations in figure 7.
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Figure 5. The geographic expansion of the feeding areas of the NE Atlantic mackerel stock as
reflected in the ICES IESSNS survey during 2007-2016. The survey runs during July and bit into
August. During these ten years the feeding area of the mackerel expanded by a factor of three
while den density doubled, indicating a six fold increase in stock size during the period. From
Nøttestad and Utne, Naturen nr 6, 2016.
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Figure 6. Trawl stations and catches of mackerel in red during the 2017 ICES IESSNS survey. Blue
bubbles are herring catches. The 2017 survey gave the highest spawning stock biomass index
ever of mackerel at 10.3 million tonnes, demonstrating the continued stock growth from 2016.
As seen from the map there are large areas in the North Sea, Norwegian fjords, Skagerrak,
Kattegat and around the British Isles not covered by the survey, indicating that the survey
produces an under estimate. Furthermore, experienced Faroese and Norwegian skippers who
have participated in the survey are worried because they see large amounts of mackerel
escaping under the trawl, thus not ending up in the spawning stock biomass index.
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Figure 7. Catches of spawning mackerel during the IESNS surveys in the Norwegian Sea May
2008-2016. Spawning mackerel was never observed in this area before 2008 but has intensified
strongly after, both in the Norwegian Sea and in Norwegian fjords way north-east at least to the
Norway-Russian border. From ICES WGWIDE 2017.
Overlapping migration routes of mackerel and southern postsmolts
Mackerel and southern European salmon postsmolts both use the shelf edge currents west of
the European continent to speed up their northern feeding migration in late spring and early
summer. Comparing figures 5, 6, 7 and 8 the geographic overlap between mackerel and
postsmolts in late spring and summer is evident.
This ‘co-swimming’ of mackerel and salmon postsmolts during the ‘on average’ about 2,000-
kilometre migration from southern European salmon river mouths to north of the Vøring
plateau in the Norwegian Sea at 68 degrees north thus creates the perfect predation
opportunity for the starving mackerel on the now slow-growing and more vulnerable
postsmolts.
Knowing that the migration takes about two months, I leave it to the reader to consider what
the effect of the combined effect of competition and predation from mackerel could be today
on postsmolts from waters off the island of Ireland, France, Portugal, Spain and western
Scotland during this migration period. Mackerel is also abundant in the North Sea and a
comparable situation would apply to postsmolts from Wales, England and eastern Scotland.
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Figure 8. Approximate swimming path of a Corrib postsmolt (red) from western Ireland to its
Norwegian Sea feeding area (yellow line). Yellow bubbles are catches of postsmolts made in
dedicated salmon trawl hauls during 1991-2011 using the Salmon-Trawl. Note how the
postsmolts follow the shelf edge current northwards then spread out in the Norwegian Sea
feeding areas, all in parallel with the mackerel as can be seen in the maps above. Original map
modified by author.
The salmon stocks in the southern NEAC area have collapsed at much higher and more alarming
rates than the Norwegian salmon stocks (Figure 9), despite about 1.3 million tonnes of salmon
and rainbow trout being farmed in Norway and only about a total of 200.000 tonnes being
farmed in two of the southern regions, Ireland and western Scotland, plus 500 tonnes in
Northern-Ireland. All of the southern postsmolts have to ‘co-swim’ northwards with the now
very dense concentrations of mackerel, more than double the distance and period compared
with the average Norwegian postsmolt.
That said, this is not to defend today’s fish farming practices which I believe are unsustainable
and should change to closed or semi-closed containment systems both from an
environmental perspective and not the least in terms of sustainable growth potential for the
industry. Sea lice is a factor for the marine survival of salmon in some areas but it is a relatively
small factor today, and should not be the point of focus in the recovery of European salmon
stocks.
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Figure 9. Development of salmon nominal catch in southern and northern NEAC 1971 to 2016.
Text at top inserted by author. Filled symbols and darker line southern NEAC.
Figure 10. Examples of the young mackerel currently growing up ‘all over’ the North Sea,
Norwegian Sea and along the Norwegian coast at the moment. These were caught in a ‘washing
set’ by the purse seiner ‘Brennholm’ at an arbitrary position 100 nm west of the Lofoten Isles in
January 2018. At this stage these small mackerels are competitors to the postsmolt salmon,
later they will be both competitors and potential predators. The new and abundant availability
of juvenile mackerel in the multi sea winter salmon feeding areas may be a good explanation to
why the MSW fishes have such a good condition at present despite their poor early sea growth.
Photo JC Holst.
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Following two years of successful mackerel spawning in 2016 and 2017 in the Norwegian Sea
and Norwegian coast, these areas are now ’full’ of juvenile mackerel (Figure 10). Consequently,
the worst may well be yet to come for the salmon from the southern European salmon regions.
The linked sea bird collapse
In parallel with the European salmon collapse, we have witnessed a collapse of a large range of
western European sea birds depending on plankton and fish larvae/small fish as main
components in their diets. The worst hit species is probably the surface feeding kittiwake, a
small sea gull, which compete directly with the mackerel in its diet. The collapse of the kittiwake
has happened in parallel with the mackerel outburst.
At the same time we have seen a strong growth in northern gannets populations, a sea bird
eating large fish and with mackerel as an important part of it’s diet. It seems nesting locations
are restricting their population growth in the western Scottish sea bird cliffs while they have
expanded strongly for example in the North Sea on Bass Rock and northwards along the
Norwegian Coast and at the Bear Island. The gannet established at the Bear Island few years ago
with a 100% nesting success and the colony is growing quickly.
In general sea birds competing for food with mackerel are plummeting in parallel with the
growing mackerel distribution and density while sea birds eating mackerel are thriving from the
dramatic increase in mackerel availability (Figure 11).
Mackerel, postsmolts and kittiwake have a large overlap in diet, in particular in amphipods, fish
larvae and small fishes. The usual explanation for the collapse of the plankton and small fish
dependent sea birds is climate change and fisheries. But it is a large paradox that the plankton
these sea birds eat is not fished and neither are 0-group fish. But both plankton and 0-group fish
are both prime elements in the mackerel diet.
Today sport fishermen see the new immigrant mackerel eating out the unfished local sand eel
stocks when they wade out on sand banks in the Lofoten area in northern Norway to fly fish for
salmon and sea trout. Earlier these stocks of sand eel were important food for salmon, sea trout
and sea birds like kittiwakes. Now they are fading away under the strong predation pressure
from mackerel, a species hardly seen in northern Norway before 2008.
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Figure 11. The strongly contrasting development in kittiwake and northern gannets on the
Runde seabird cliff off the Norwegian west coast at about 62° north is a good example of how
the plankton and small-fish eating sea birds lose out in the competition with the mackerel while
the gannet, a mackerel eating sea bird, prosper from it. Following the track of the mackerel
outburst from Ireland north to Spitzbergen we can find a comparable pattern repeated over and
over again in various sea bird cliffs.
Reasons unknown
Some scientists claim temperature and climate change is the culprit for the European wild
salmon collapse. In my view there is no empiric basis for such a conclusion.
If we study water temperatures in the main feeding area of ‘southern’ European postsmolts in
the Norwegian Sea, they rose from the 1970s to 2007 and have now dropped to close to or
below normal, according to the Institute of Marine Research (IMR) in Norway.
Figure 12 below describes the development in temperature conditions in the most important
feeding areas of European postsmolts during the late spring-summer.
Climate variability
Temperatures in the Norwegian Sea follow the so-called Atlantic Multidecadal Oscillation
(AMO). This 60-year climate cycle bottomed in the early 1970s, peaked around 2007 and is
expected to be negative over the next 20 years from now.
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Climate change
Climate change will probably lead to higher temperatures at the peaks and troughs of the
coming cycles, but I expect the about 60 years AMO cycling to continue as for instance
documented in sedimentation layers on the seabed since the last ice age of 10,000 years ago.
So, during a period of continuous decline of salmon stocks from in particular the southern
European area from around 1973, temperatures in these main feeding areas for European
postsmolts have been rising and peaked in 2007 and have dropped to around normal today.
Consequently, there is no correlation with temperatures and the collapse of the southern
European salmon stocks but there is a very good correlation with the growing mackerel stock
and its potential for competition with and predation on the European postsmolt salmon.
Figure 12. From the Institute of Marine Research report 2017: The Norwegian Sea: The
temperatures in the Atlantic water along the Norwegian continental shelf have since 2013 been
close to or slightly above normal. The temperatures in 2016 were mainly above normal, except
the south-eastern Norwegian Sea were the temperatures were lower than normal.
An international thinning fishery for mackerel
Despite the AMO having turned negative more than 10 years ago European salmon stocks
continue their negative spiral and fishing has almost ceased as the stocks are close to or under
conservation limits in particular in the southern NEAC area.
In my view, this situation will probably continue to worsen until the heavy competition and
predation from mackerel is reduced. The reduction should in my view be done through an
internationally agreed and closely monitored thinning fishery on mackerel, where some of the
extra catch goes into reduction to meal and oil.
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Not to give the pelagic fishermen higher quotas but to bring the ecosystem of the Norwegian
Sea and neighbouring seas back within ‘normal’ ranges where both salmon and seabirds
dependent on plankton and small fishes will return to sustainable stock levels.
Is it risky to actively reduce the mackerel stock?
It is claimed that such a thinning fishery will be too risky. I do not agree in this view. By closely
following some primary indicators directly linked with the mackerel stock on a running basis one
will be able to evaluate almost instantly the effect of a lowered mackerel stock and the lowered
grazing pressure from it. Such primary indicators will be the individual growth of the mackerel,
the size of the mackerel distribution area and the concentrations of the main food of the
mackerel in the Norwegian Sea, the copepod Calanus finmarchicus. Once the stock becomes
closer to or within sustainable levels the thinning fishery would be reduced and eventually
stopped.
Somewhat delayed one will also start seeing positive changes in more secondary indicators, like
for instance in stock levels of amphipods, sprats and sand eel, kittiwake breeding success,
recruitment in the Norwegian spring spawning herring and postsmolt salmon growth and
survival. One good secondary indicator will be the ratio of grilse over multi sea winter salmon in
salmon stocks, which will start increasing quickly from today’s record low levels as soon as the
food availability for postsmolt salmon improves as a result of lowered competition from
mackerel. Changes in the secondary indicators will be delayed because they are not directly
linked with the mackerel stock and because these indicators are more slow growing species,
depending on more time to rebuild sustainable stock levels than for instance the short lived
Calanus finmarchicus.
Salmon bycatch in a mackerel thinning fishery?
Another worry is the possible bycatch of salmon such a thinning fishery for mackerel could
cause. Based on the present knowledge about the seasonal overlap between different salmon
ages and mackerel it will in my view be possible to advice areas and periods for such a thinning
fishery that would minimize bycatch of salmon and other fishes. If some bycatch of salmon
should occur it will in any case be worthwhile due to the large potential of increased salmon
stocks that the mackerel stock thinning fishery would lead to.
Looking at figure 9 I will grossly estimate a ‘normal range’ for the southern NEAC salmon catch
to be in the range of 2000-3000 tonnes while the corresponding northern catch would be 1500-
2000 tonnes. In my view the early 1970ies salmon catches were over the ‘normal range’ due to
the late 1960ies herring and mackerel collapse. Both herring and mackerel being out of the
Norwegian Sea ecosystem during the early 1970’ies led to an ‘un-natural’ situation with
extremely high food levels and potential low predation levels from mackerel for the European
postsmolts.
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Conclusions part 1
The hypothesis on overgrazing and predation has been hard to sell within the ICES community,
in management bodies, with politicians and with the wild salmon lobby. The reason for this is
that the hypothesis challenges one of the cores of ICES activities, the accuracy of pelagic stock
estimation. It also challenges the hypotheses that sea lice and escaped farmed salmon are the
most serious threats for the wild salmon today. Nevertheless, in search of getting our salmon,
sea birds and high quality mackerel for the markets back, every obvious stone must be turned.
I have worked with a wide range of mortality factors on wild salmon at sea since 1991, including
sea lice, disease and bycatch. It is my strong recommendation that also this hypothesis should
be treated and tested seriously. The empiric basis is very much larger than can be shown in this
handout.
The ocean is something large and unknown for most of the salmon, sea bird and mackerel trade
communities. I think this is the main reason why for example most discussions on understanding
and managing the wild salmon focus around rivers and near shore issues. The time is overdue to
look out at sea through an ecosystembased perspective. I will guarantee an interesting and
intriguing journey that will give you an entirely new view on why the salmon and sea birds vary
in numbers and why they are not as numerous as they used to be. And why the G6 mackerel is
gone and the remaining mackerel of poor quality today. Let’s start today and bring back all of
them, like they used to be and like they should be in a sustainably managed ecosystem!
So I ask: What is most likely to kill a northward bound 15 cm Corrib postsmolt salmon today
temperatures close to normal or a starving mackerel?
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Chapter 2
Why is the 2018 ICES assessement and quota advice on NE Atlantic mackerel a gross
underestimate?
The mackerel explosion which never turned up in the mackerel quotas
In an article in the Norwegian popular science magazine "Naturen" No. 6/2016,
( https://www.idunn.no/natur?languageId=2#about ) the stock responsible scientist on
mackerel at the Institute of Marine Research, Norway (IMR), Leif Nøttestad and management
scientist Kjell Rong Utne at IMR writes: "The mackerel stock has undergone an incredible growth
in the amount, extent and density over the past ten years. It has been documented how the
mackerel has gradually spread northward in the Norwegian Sea and into neighbouring sea and
coastal areas. The mackerel stock is now distributed within many countries' economic zones -
Norway, the EU, the Faroe Islands, Iceland and Greenland, as well as international waters. Since
the mid-2000s, the spawning biomass of mackerel has risen sharply. During the same period, the
mackerel has increased the distribution during the feeding period in the summer from 1 million
square kilometers to over three million square kilometers, and had a doubling of density from
1.5 to 3 tonnes / km2. The main reason for the formidable increase in the distribution of
mackerel over the last decade is assumed to be the sharp increase in stock size. There has been
historically strong recruitment over the past ten years, including five of the strongest year
classes ever recorded. A large mackerel stock needs much more space than a small mackerel
stock. In addition, there is great competition for limited food resources, which causes the
mackerel to migrate further north and west in the summer in search of food." (Figure 5).
Since this article was written in 2016 the ICES International Ecosystem Summer Survey in the
Nordic Seas (IESSNS) in the Norwegian Sea and adjacent seas measured the largest spawning
biomass index ever at 10.3 million tonnes of mackerel. Accordingly, the amount of mackerel
today must be considered as larger than what the article describes. As mentioned earlier
another result of the stock ‘explosion’, the mackerel has expanded its spawning areas from
mainly west of the British Isles and the North Sea to large parts of the Norwegian Sea from the
Faroe Islands and across to the Norwegian coast to the northernmost part of Norway (Figure 3,
4 and 7). In 2014, 2016 and 2017, the recruitment of mackerel seems to have been successful in
these areas, and today large parts of the Norwegian Sea and many Norwegian fjords are well-
filled with young mackerel (Figure 10). This young mackerel comes in addition to the grown-up
mackerel also grazing and spawning in these northern areas. The mackerel's expansion in the
north-east Atlantic was in no way complete when Nøttestad and Utne wrote their article in
2016. The growth has continued and we have probably not have seen the most serious
consequences of it yet.
20
In summary, according to the IMR, the mackerel distribution is currently over three times as
large as ten years ago, and the density has doubled. Accordingly, based on the record
measurement from the IESSNS survey in 2017, there must be at least six times more mackerel in
the NE Atlantic today than in 2007. With at least a sixfold increase in stock size over the past ten
years in mind, the question mark arises when we read the ICES 2018 Mackerel Advice, the "ICES
Advisory Sheet 2017". In the advice we can see that ICES estimates that the spawning stock in
2008 was 2.8 million tonnes (Figure 13). In the quota advice for 2018, ICES estimates the
mackerel spawning stock to be 3.1 million tonnes in 2018, an increase of approx. 10 percent
from 2008. In view of Nøttestad and Utne, claiming that the stock has increased by a sixfold it is
not understandable how the spawning stock estimate and quota advice has only risen by 10
percent? If we multiply the estimated spawning stock in 2008 of 2.8 million tonnes with six, as
indicated by the trawl survey, we will get a spawning stock in 2018 of 16.8 million tonnes. This is
a spawning stock significantly more in line with what the fishermen experience at sea and the
reality in the ocean in my view.
Figure 13. Spawning stock development of the NE Atlantic mackerel stock. The red circle is the
spawning stock size for 2018 as projected by ICES. Inserted by the author. From ICES ADVICE
2017.
Furthermore, in 2008, the spawning stock was in my view probably largher than 2.8 million
tonnes, a view supported by calculations made by scientists at the IMR which we will look into
further down.
21
So how did we end up in a situation where all scientists, sports- and professional fishermen
observe that the ocean is filling up with more and more mackerel, while the stock estimate and
quota advice of ICES remain close to constant?
Mackerel stock estimation and stock assessment
Today three methods are used to estimate three independent spawning stock indexes of
mackerel: the egg method, the trawl method and the tag method. All three stock indexes are
included in the stock assessment of the NE Atlantic mackerel stock from the 2017 advice. I will
first present a very brief overview of the three methods, then I will evaluate how the spawning
stock biomass index coming out of each of the three methods are used and handled in the
mackerel assessement today.
The three methods can be explained very simplified as follows:
Egg method: By hauling a plankton net through the water at many different positions supposed
to cover the entire mackerel spawning period and spawning area, it is estimated how many
eggs the mackerel stock have spawned in a certain year. This number is used to estimate how
many female fishes are required to produce these eggs, and then this number is multiplied by
two to account for the males. Then this number is multiplied by the estimated average weight
of all spawners which gives a biomass index for the spawning stock of mackerel.
Triannual egg surveys started west of the British Isles in 1992 and the last survey in this area
was in 2016. There is also been a parallel egg survey in the North Sea.
The trawl method: It is systematically trawled with the same trawl and trawling method over an
oceanic area that is supposed to contain the entire spawning stock of mackerel in the summer.
The catches in the individual hauls are distributed on the trawled area, and the average density
of mackerel per square kilometre is calculated. Then this density is multiplied with the size of
the entire area covered by the survey which gives an index for the total spawning stock
biomass.
The tagging method: Mackerel is tagged in spring west of the British Isles. Based on the
numbers of fishes tagged, knowledge about the tagging mortality, detection of tagged fish in
commercial catches and the size of these catches it is possible to estimate the size of the
mackerel spawning stock.
Mackerel has been tagged for migration studies and stock assessement purposes yearly west of
Ireland and Scotland in May starting in 1969. There are two tagging series on mackerel now
used in the ICES stock assessment. The first series was started in 1969 using magnetic steel tags.
It ran up to 2007 when the series was ceased amongst other because of problems with
collecting tags efficiently from the fish plants. A new series was started in 2011 using so called
electronic RFID tags. In the steel tag experiment from 5600 to 34000 mackerel were tagged
22
annually. Between 2011 and 2017 on average 52.260 mackerel have been tagged electronically
per year. The steel tag series is only used for the years 1980-2005 in the 2017 assessement and
electronic tag recaptures from 2012 to 2016.
The electronic tags are detected automatically when a tagged mackerel passes through an
electric field mounted around the delivery bands at 17 fish plants in Denmark, Faroes, Iceland
Ireland, Norway and Scotland. When the tag inside a mackerel passes through the electric field,
a voltage builds up in a copper coil inside the tag. This voltage makes the electronics in the mark
transmit a unique identification code. The code is detected in the electric field and are sent
continuously to the IMR tag database via the net. In addition to the tag identifiers, it is
important for the IMR to receive reliable information about the amount of mackerel that passes
through the tag detectors at the various plants.
Evaluation of the use of the three stock biomass indexes in the stock assessement
The egg method:
The spawning stock biomass index which came out of the egg investigations for western
mackerel in 2016 was 3.8 million tonnes. I will argue that the egg method produces a spawning
stock index that systematically underestimate the biomass of spawning mackerel. One obvious
explanation to this is that the survey only covers the area from Portugal to the west of the
British Isles north to between Shetland and the Faroe Islands (Figure 4). Thus, the egg
production is lost over a large sea area in the Norwegian Sea and along the Norwegian coast,
and thus a lot of the spawning stock in the spawning stock biomass index (Figure 7).
In the 2016 executive summary of the ICES Working Group on Mackerel and Horse Mackerel Egg
Surveys it is stated: “However, analyses showed that the mackerel core spawning area was
covered and a reliable estimate of mackerel annual egg production was delivered.” Given the
available knowledge that mackerel has spawned in the Norwegian Sea since 2008 (Figure 7) it is
not understandable how the ICES working group can claim they have worked up a reliable
estimate of the annual egg production of the mackerel stock in 2016.
In the Norwegian fisheries paper ‘Fiskeribladet’ of 26th May 2016, the head of the pelagic
section at IMR, Aril Slotte, states in an interview that “We are looking into expelling the 2016
egg survey results for mackerel as it appears that not the whole spawning stock was covered”.
Obviously the ICES scientists are aware of this serious bias in the egg survey and are willing to
start omitting the egg data from the assessement. This opens a totally new view on the
mackerel assessment where the IESSNS survey and the tags will play a much more important
role.
Apart from the area and periods not covered by the survey there is something more and
inexplicable in the egg method that strongly underestimate and smooth out the stock variations
that are obviously occurring in the spawning stock of mackerel (Black squares in figure 14). In
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my view, this can be due to density-dependent spawning over time, horizontally and vertically.
That is, the mackerel regulates the spawning density in a way we are unable to capture in the
estimation of egg production. In this way, we lose more and more eggs both horizontally,
vertically and temporally as the stock increases and needs more time, area and depth to carry
out the spawning with about the same density of fish. Another possible explanation can be
strong predation on the eggs that is not taken account of in the estimation of egg production.
There may also be other reasons to the underestimation of the egg production.
Figure 14. The figure shows various stock indexes and estimates on the NE Atlantic mackerel
produced by ICES. The green line is the spawning stock size history as estimated and presented
by ICES in the 2017 stock advice. The red diamond is the 2018 spawning stock projection for
2018 taken from the 2017 ICES stock advice. The black squares is the spawning stock biomass
index coming out of the triannual western egg survey starting in 1992 as presented by ICES
WGWIDE in 2018. The blue line is the biomass index coming out of the IESSNS trawl survey as
presented by ICES WGWIDE 2017. The yellow line is the spawning stock development coming
out of the tag experiments when equal tagging mortality is estimated for the steel and
electronic tags. The first part of this dataseries is from figure 16 in this document. The latter
part, after the curve goes outside the y-axis of the graph is reconstructed using the F values in
the right panel of figure 16.
24
It is well documented that the mackerel quota was heavily overfished for many years by
different nations without the stock collapsing. During this period the egg survey was the only
spawning stock biomass index. This further strengthens the view that the egg method has
systematically underestimated the mackerel stock for many years.
The trawl method
The scientists who participated in the ICES Mackerel Benchmark in 2017, did not appreciate the
high estimates emerging from the IESSNS surveys. The high numbers simply do not fit into the
assessement model.
In the general discussion of the results of last year's trawl record, the scientists write on page 9
in ICES WKWIDE 2017: «The estimates in absolute terms are likely to be biased (upwards,
potentially by a factor of 2) due to the use of wing swept area in the estimation of fish density.
Mackerel are likely to be herded by the boat, the doors, and the net, and although an
appropriate factor may be difficult to determine, the use of door spread, which represents the
larger area sampled would provide a more conservative estimate. This point is all the more
compelling, given that the catchability of the survey from the assessment is 2, and the wing
spread is half the door spread (i.e. if door spread was used, the survey estimate would be half of
what it is, and the catchability would be 1).”
So because the stock estimate from the trawl survey is larger than the scientists had
anticipated, given the prior view on the stock size based on the egg survey, the scientists explain
the large biomass index from the trawl survey with the trawl probably catching twice as much
mackerel as what was in the water mass sifted through the meshes of the trawl. This means that
they are of the opinion that the vessel, the doors and the sweeps collect the mackerel in front of
the trawl itself. But this view is ‘taken out of the air’ when suddenly the trawl survey give a large
result conflicting with their prior much lower view on the spawning stock based on the egg
survey.
It is consequently a very critical point how large the effective catching width of a trawl is (Figure
15). In my view the efficient catch width of the trawl should be determined by independent
methods, not as a result of a survey result which is ‘found to be too large’ given the results
coming out of the egg data series (Figure 14).
The assumption is that the IESSNS trawl survey overestimates the spawning stock by a factor of
two is in sharp contrast with the assessments of former foreign and Norwegian skippers on the
IESSNS survey. (Today's Norwegian skippers have signed a confidentiality declaration).
According to these, very much of the mackerel disappear below the trawl and is not caught.
Much of the mackerel in an area therefore does not appear in the trawl catches and
subsequently not in the trawl survey spawning stock biomass index.
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Figure 15. What parts of a trawl are actually collecting the mackerel and how wide area does a
pelagic trawl catch mackerel from? This number is critical to the biomass index coming out of
the IESSNS trawl survey. When the ICES WKWIDE scientists get a biomass index from the IESSNS
trawl survey that is too large in their opinion, they choose to use the trawl door distance (2xA in
this figure) to calculate the density of mackerel in the ocean instead of the trawl wing distance
(A in this figure). This halves the density of mackerel in the ocean and halves the biomass index.
No pelagic trawl skipper would agree that all the mackerel passing between the trawl doors
would end up in the trawl cod-end.
They are also of the opinion that the semi-circular trawl method, the so-called banana haul,
used in the survey is destroying the trawl's normal symmetry and fishing efficiency. They claim
that by trawling in a semi-circle less fish is caught than when trawling the same volume of water
straight ahead. No commercial fishing boats living from their catch would trawl in a circle the
way the research vessels do in this survey. In one case, a survey vessel was allowed to trawl
straight ahead in the same area as it had trawled in semi-circle just before. These two straight
hauls each gave about a double catch compared to the banana haul.
On my own account I would like to add:
a) No corrections are made for the mackerel trying to swim out off and along inside with the
trawl during the capture phase. Even though the trawl is hauled at five knots, ie approx. 2.5
meters per second, the mackerel will keep up with the trawl for a while before it is exhausted
and ends up in the cod end. Much of the mackerel inside the trawl belly when the vessel stop
towing and start hauling the trawl will consequently swim out of the trawl as the speed of the
trawl is strongly reduced at that stage. Summing up, the trawl does not capture all the mackerel
that was in the water mass sifted by the trawl meshes before the start of the haul. To
compensate for the swimming of the mackerel during the haul and those swimming out when
the towing stops, the catch should be multiplied up by a factor greater than 1. This is not done.
b) There are large areas of mackerel that are not covered by the trawl survey. This applies to the
North Sea, the Norwegian fjords, Skagerrak, Kattegat, the Danish belts and the areas around the
26
British Isles (Figure 6). Reports from various sources throughout this area tells that there are
large amounts of mackerel also in these areas not covered by the IESSNS trawl survey.
Based on the factors above it is my opinion that the spawning stock biomass index from the
trawl survey in 2017 at 10.3 million tons is a large underestimate of the real spawning stock size.
The ICES scientists, however, decides based on feelings that the trawl survey overestimates the
spawning stock by a factor of two. Therefore it is decided to double the efficient trawl width
from between the wings to between the doors which halves the spawning stock biomass index
coming out of the trawl survey.
The tagging method
In the case of stock estimation with tags, the tagging mortality is a fundamental size. "Tagging
mortality" means the additional mortality the actual tagging process causes the mackerel.
Important causes may be mortality due to so-called osmotic leakage, another cause bleeding
and a third that the tagged mackerel is eaten by predators after it has been released from the
vessel.
Osmotic leakage means that freshwater flows through the skin where the mackerel has lost
scales during tagging. If this leak grows larger than the amount of freshwater the gills can pump
into the fish, it will slowly, but surely get saltier and saltier inside, dry out and eventually die. If
the loss of scales is at the limit of what the fish can withstand, this process will occur within days
after the tag was inserted. If the scale loss is large, it will die quickly. When tagging herring and
salmon it is easy to see how much scales a fish has lost, thus avoiding tagging individuals who
have excessive scale loss. For mackerel, it is harder to see this, so that the handling and
assessment of individuals becomes even more important. The tagging process on the IMR
tagging surveys is led by highly experienced and skilled personnel, who do what they can to
make the tagging process as gentle as possible. Among other things, the individual mackerel is
considered carefully before tagging and bleeding mackerel is not tagged. Earlier, the tagged fish
were released on the lee side of the vessel, ie up against the wind. This gave the gannets good
conditions to hang over the area the mackerel was released into, then to dive down and eat the
tagged mackerel.
In connection with a new tagging procedure introduced in 2006 the fish is now released
downside the wind. The mackerel is also released through a hose running under the gannet
diving depth. These changes makes it almost impossible for the gannets to eat the tagged fish.
Therefore, the mortality rate due to predation from the gannets must be considered to be
significantly less today than before the new procedure was introduced.
The pensioned skipper Lodve Gjendemsjø on the Norwegian purse seiner Inger Hildur who has
participated in 17 herring and 3 mackerel tagging surveys for the IMR told me “I participated in
tagging surveys using both the old and the new tagging procedure on mackerel. I consider the
new one to be much better than the old one and the tagging mortality must have come down a
lot.” He listed the following reasons for his view: a less intrusive handling of the mackerel,
round keep tanks instead of rectangular ones allowing the mackerel to swim in circles rather
27
than swimming into the corners of the tank and less predation by gannets du to release of the
mackerel downside the wind and through a hose.
Tagging data in the mackerel stock assessment
During the ICES Mackerel benchmark in February 2017, the ICES stock assessement model SAM
was adapted to include the two tag data series. Several runs were made with different settings.
Amongst other, a run where the model used equal tag mortality for the two tag series. This
resulted in a stock estimate so high that the number was outside the Y-axis of the standard
graph used in the report. In Figure 16 this is the light blue line that "disappears through the
roof". About this run the benchmark scientists say in the WKWIDE report on page 41: «The
model assuming a single survival rate and overdispersion was clearly not appropriate,
estimating unrealistic SSB (spawning stock biomass) and Fbar values (Figure 3.3.4.1). The post-
release survival rate estimate, mostly representative of the steel tags which represent the bulk of
the data, appears to be too high for the RFID tags and the model can only deal with the lower
recaptured numbers in this dataset by estimating large stock-abundance for the corresponding
cohorts. » The stock estimate for 2016, which is not shown in the graph because the Y axis is too
short, can be estimated to be around 22 million tonnes (Yellow line in figure 14). This figure was
considered to be too high by the scientists which describes the estimate as "clearly not
applicable, estimating unrealistic SSB and Fbar values".
Figure 16. When the WKWIDE benchmark scientists made a model run assuming equal tag
mortality for the old steel tag series and the new electronic tags the estimated spawning stock
passed up ‘through the roof’ of the standard graph, light blue line in the figure. At the same
time the fishing mortality, F, in the right graph dropped down to 0.07. The scientists did not like
this high stock estimate and rejected it as ’unrealistic’.
28
Again the scientists reject a result emerging from the observational data based on feelings. As
the model run assuming equal tag mortality between the old and new tag series gave a “clearly
not appropriate, estimating unrealistic SSB” they decided to overlook the result and run the
model with unequal tag mortalities which heavily takes down the estimated spawning stock
from the electronic tags, to level it with the egg biomass index.
The scientists then ran the model so that it should estimate separate tagging mortality for the
steel tags and the electronic tags. In order for the equation to "go up", the model came out with
a tagging mortality at 61 percent for the steel tags and 92 percent for the electronic tags. This
means that the scientists accepted that 92 out of 100 mackerel tagged with electronic tags dies
as a result of the tagging. And the explanation? This gave a much smaller stock estimate which
they considered acceptable.
Then, in the WGWIDE 2017 working group, the scientists accepted an estimated tagging
mortality of 91 per cent after the same principles as described in WKWIDE 2017 above.
However, in an interview in Norsk Fiskerinæring, May 2018, the head of the mackerel tagging
project and leader of the pelagic section at IMR Aril Slotte says "The estimated tagging mortality
of over 90 percent is not understandable in my eyes. It is also difficult to understand for our
tagging staff who assess the quality of mackerel before tagging. Consequently, I assume that the
tagging mortality is not as big as 90 percent. But to assume does not hold. We must document
with scientific methods and data." The latter sounds very reasonable to me. However, when it
comes to rejecting equal tagging mortality between the two tag series, it is enough to assume
that they must be different to reject the high result. Although the staff, who have long
experience in sorting the mackerel before it is tagged, say that they use exactly the same criteria
to sort the mackerel as before.
How high is the real tagging mortality?
Two experiments on tagging mortality in mackerel has been carried out. One by Hamre (Hamre,
1970 ) and one by Lockwood (Lockwood et al., 1983). In Hamre’s experiment 100 internally
tagged mackerel and an untagged control group of 100 were kept in a keep net together for
three weeks. The mortality rate of the tagged mackerel was 18% and the control group
mortality was 9%. In Lockwood’s experiment 93 tagged and 92 untagged mackerel were kept in
a keep net for 15 days. The mortality of the tagged group was 18.3 % and control group survival
was 4.3%. There were not predators around in these experiments but one could for instance
think of negative ‘laboratory’ effects on the survival rate related to the keeping of the mackerel
in net pens for up to three weeks in both these experiments. Such effects would not apply to
the ‘at sea tagging’ where the mackerel is back at sea within very short time after being fished
and tagged.
An article by Tenningen, Slotte and Skagen from 2010 (Tenningen et al., 2010) deals with testing
the tagging method for mackerel in stock estimates based on the old steel tags. Compared to
how benchmark and WGWIDE 2017 treat the tagging results, the conclusion in the paper is
remarkable: «The spawning stock biomass estimates derived from two different tag-based
29
models were highly variable, but were on average 2 and 2.3 times higher than the ICES official
estimate. The official estimate is considered uncertain and most likely to underestimate the
actual biomass due to unregistered fishing mortality and lack of fishing-independent, age-
disaggregated data. Hence, tag-based estimates could potentially improve the current
assessment if included in the ICES stock assessment on a regular basis. These estimates also
involve some uncertainty that needs assessment, especially related to variable tagging
mortality, detector efficiency and migration of the stock.» In this article, tagging mortalities at
30, 40 and 50 percent were used, which is very far below what the scientists accepted in the
mackerel advice for 2018 at 91%.
When the ICES scientists in WKWIDE and WGWIDE accepts a tagging mortality at up to 92% to
get the model equations ‘to go up’ with the catch and egg data it is simple and plain
manipulation with data and not science in my view. It is simply a totally unacceptable way of
working for ICES.
So, in general when the ICES scientists get a stock biomass estimate from the IESSNS trawl
survey or the tagging experiments that doesn’t fit into their prior perception of the stock, they
simply do some massaging of the data series and come up with lower spawning stock indexes
that fits better with their prior perception of the stock size. This prior perception is mostly based
on results coming out of the egg data series, a series they admit underestimates the spawning
stock and which they are ready to partly exclude from the assessment according to Aril Slotte.
Again, this is not a scientifically acceptable way of working within ICES in my view.
How large is the mackerel stock today?
As we recall from the chapter on the mackerel explosion that disappeared, the observations
from the trawl survey indicate at least a sixfold increase in the mackerel stock from 2008 to
2018, from 2.8 to 16.8 million tonnes. 16.8 million tonnes is not far from 22 million tonnes, as
the tagging series indicate using equal tagging mortality the steel and electronic tags. Again,
based on all available information, I will strongly claim that at 22 million tonnes we are
significantly closer to the actual size of the mackerel stock than the 3.1 million tonnes ICES
estimated for 2018 (Figure 14).
If we assume that the tagging mortality of the two tagging methods is actually about the same
and that the run of WKWIDE 2017 is both 'appropriate' and 'realistic', we will get a stock
development that starts to increase sharply from approximately 2003 and which reaches
somewhere above 20 million tonnes in 2018. Such a development will suit the development
described by Nøttestad and Utne based on the increase in distribution and density of the stock
as indicated in the IESSNS trawl survey (Figure 5). With such a solution to the equations, the
map begins to fall in place relative to the terrain.
The whole story becomes even more incomprehensible and less consistent if we look at the tag-
based stock estimates in Tenningen and others (2010). In this paper it is estimated that the
mackerel stock based on the steel tags was just over 7 million tonnes in 2005 and 2006 (Figure
17). If this is our starting point at that time and we multiply with 6, we end up with an estimate
30
of the mackerel stock of around 40 million tonnes in 2018. This biomass is well in line with what
an experienced skipper who has participated in the trawl surveys previously claimed in a
conversation: "After what I've observed and experienced in the mackerel surveys, the mackerel
stock can well be between 40 and 50 million tonnes."
Figure 17. Stock estimates of 3-12 years old mackerel, 1986-2006, based on the MERKAN and
HAMRE tagging models. The estimates are compared with the official stock estimates (ICES,
2009) and the triannual egg survey (ICES, 2008). The estimates circled in red are at about 7
million tonnes are the MERKAN og HAMRE estimates for 2005 og 2006 as mentioned in the text
above. From Tenningen et. al. 2010.
Summary about the mackerel assessment
Today we have three biomass indexes of the mackerel stock, all of which all indicate a mackerel
stock significantly larger than the ICES forecast for the 2018 spawning stock at 3,1 million tonnes
(Figure 18). The egg data indicates a stock of 3.8 million tonnes, the trawl survey a stock at 10.3
million tons and the tagging data a stock around 22 million tons given equal tagging mortality.
But the ICES scientists have no doubt that the spawning stock index at 22 million tonnes
emerging from the tagging experiments is too high and that the 10.3 million tonnes trawl index
overestimates the stock by a factor of 2.
31
Figure 18. Starting out with three spawning stock biomass indexes at 22, 10,3 og 3,8 million
tonnes the ICES WGWIDE stock assessment ends up with a stock assessment at 3.1 million
tonnes. In addition to the three spawning stock biomass indexes the assessement model uses
catch data and a recruitment index as input.
How, then, can the model come out with a stock estimate that is even significantly less than all
the three spawning stock biomasses? In addition to the fact that the scientists reduce the size of
the indexes they find ‘too large’, the assessment model is programmed to weigh the biomass
indexes inversely relative to the size of the standard deviation (variation) to the individual data
series. This means that little variation in a data series gives high weighting, large variation gives
low weight. Because the catch data and egg biomass index have low variations, these data sets
get high weights in the model result while the trawl survey and the tags have high standard
deviation (high overdispersion for the tags), and consequently gets low weight and little
significance for the model result. This weighting is done without at all discussing how accurate
the different series are; that is, how well they hit the real spawning stock size in the ocean. And
no independent investigations are carried out to try to find out about this
Thus, in the process of approving the size of the various stock biomass indexes used in the
model, the rules vary according to what the scientists find an acceptable size. If an index is
found too large, given the existing view of the stock, it is adjusted down to fit better. Then the
statistics in the model take over and determine what weight a series should have: Low standard
deviation, high weight. High standard deviation, low weight. However, the extent to which the
individual biomass index hits in relation to mackerel in the ocean is not discussed and does not
affect the size of the assessement or quota advice of ICES.
32
So what accuracy does the final quota advice has in relation to the real stock of mackerel at sea?
In my view absolutely nobody know this today. The only thing that is certain based on the above
is that the 2017 stock assessement of ICES is far too low.
What went wrong in ICES?
Looking back at the ICES stock assessement and advisory story on mackerel we see that the egg
investigations has been the cornerstone, starting in 1992. With its long history it forms the basis
for the present general view on the mackerel spawning stock size with the ICES scientists (Figure
14). Then the IESSNS trawl survey was initiated in 2007 and came up with a new and higher
stock biomass index. After that the new electronic tagging series was initiated in 2011 and was
included in the assessement from 2017, with an even higher spawning stock biomass index
given equal tagging mortality between the old and new tagging procedure.
The egg investigations always showed a very stable and low stock (Figure 14). Despite proven
heavy overfishing for many years by different nations the stock never collapsed, indicating that
the stock estimates based on the tags always was an underestimate. Now, when two new
biomass indexes indicate a much higher stock level this come in conflict with the former stock
size history estimated from the tags (Figure 14). It is in this situation the ICES scientists fail in my
view. Rather than initiating investigations to check the accuracy of the results emerging from
the trawl and tagging methods they decide to reduce the indexes based on feelings and further
down weigh them in the model using statistically based weighting.
In my view the correct scientific way to deal with this situation would be to initiate independent
investigations in particular on the efficient catching width of the trawl method applied in the
IESSNS survey and on the actual size of the tagging mortality both using the old and the new
tagging procedure. It will be fairly simple to design and carry out experiments which would give
a better insight into both the efficient vertical and horizontal sampling width of the trawl, and to
the tagging mortality of the tagging experiments. Of course, none of these two sizes will have
constant values but well designed experiments will definitely make it possible to get a realistic
grip on their real range.
The unfortunate respect for the model results
As I see it, we have had a very unfortunate development where management advice in this field
get more and more distant from what the various investigations tell about the size of fish stocks.
Instead, the scientists trust the model results more and more. This despite the fact that the
scientists are aware of weaknesses, sources of error and broken model assumptions behind the
results. These motions are poorly communicated and the model results are interpreted as truths
by managers, politicians and the public. At the end, model results almost unrooted in reality end
up as the basis for socially, economically and ecologically important management measures.
This is a very dangerous development. In more and more areas, this practice leads us to an
almost purely model-based management that is not based on the results of various
investigations and the reality of the marine areas we manage.
33
Conclusions
I started participating in ICES working groups in the late 1980’ies. Since then I have participated
in a series of groups in particular focusing pelagic fish stock assessment and ecosystem related
questions. When I go through the 2017 ICES mackerel assessment in detail, I’m shocked about
how the scientists treat the two data series indicating a sharp rise in the mackerel stock and
how this assessment has been carried out. In my view ICES need to take a big step back and
simply reset the entire mackerel advisory process. A very serious situation has arisen which has
brought a whole large ecosystem far outside its natural range of variation with grave
consequences for all its inhabitants including man as the ultimate predator and manager of the
system today.
This is a deep scientific conflict that needs to be treated as such. Not by trying to stigmatise me
and the work I’m doing. I have no other agenda than trying to get the hypothesis on overgrazing
and predation tested in an objective and neutral way on behalf of the salmon, the sea birds and
the G6 mackerel.
About the auhor
Dr Jens Christian Holst worked as a management scientist on pelagic fish at the Institute of
Marine Research in Norway. Today he is an independent fisheries advisor and developer. He
started working on the marine ecology of Atlantic salmon in 1991 and moved into general
ecosystembased management in the early 2000.
Several of his publications and talks on salmon at sea and related ecosystem issues are found by
registering and logging into Researchgate.com or searching at Scholar.com. Relevant search
words are salmon, laks, herring, sild, mackerel, makrell, Norwegian Sea and Norskehavet.
To stay independent in the controversial field of wild salmon, sea lice, farmed salmon and
pelagic fish, he has not taken on projects since 2016 and finances himself today. This handout
and all its conclusions is consequently totally his own work and not paid by anyone. He is
involved in projects on closed contained fish farming.
34
References
Hamre, J., 1970. Internal tagging experiments of mackerel in the Skagerrak and the
392 northeastern North Sea. ICES CM 1970/H:25.
ICES. 2017a. Report of the Benchmark Workshop on Widely Distributed Stocks (WKWIDE), 30
January3 February 2017, Copenhagen, Denmark. ICES CM 2017/ACOM:36. 196 pp.
ICES. 2017b. Report of the Working Group on Widely Distributed Stocks (WGWIDE),
30 August -5 September 2017, ICES Headquarters, Copenhagen, Denmark. ICES CM
2017/ACOM:23. 1111 pp.
ICES. 2017c. Report of the Working Group on North Atlantic Salmon (WGNAS), 29 March7 April
2017, Copenhagen, Denmark. ICES CM 2017/ACOM:20. 296 pp.
ICES 2017d. ICES Advice on fishing opportunities, catch, and effort Ecoregions in the Northeast
Atlantic and Arctic Ocean Published 29 September 2017 Mac.27.nea
DOI:10.17895/ices.pub.3023
Lockwood, S.J., Pawson, M.G., Eaton, D.R., 1983. The effects of crowding on mackerel
414 (Scomber scombrus): physical conditions on mortality. Fish. Res. 2, 129-147.
Tenningen, M., Slotte, A. and Skagen, D. 2010. Abundance estimation of Northeast Atlantic
Mackerel based on tag recapture data - a useful tool for stock assessment? Preprint of article in
Fisheries Research found at Brage, IMR: https://brage.bibsys.no/xmlui/handle/11250/108529
Article
Full-text available
Atlantic salmon Salmo salar is a socio-economically important anadromous fish species that has suffered synchronous population declines around the North Atlantic over the last five decades. Reduced marine survival has been implicated as a key driver of the declines, yet the relative importance of different stressors causing mortality at sea is not well understood. This review presents a synopsis of the principal stressors impacting Atlantic salmon in estuarine and marine environments. It also applies a semi-quantitative 2-D classification system to assess the relative effects of these stressors on English salmon stocks and their likely development over the next decade. Climate change and predation were identified as the biggest threats at present and over the next decade. Poor water quality and bycatch were classified as relatively high impact stressors, but with a lower likelihood of becoming more prevalent in the future due to available mitigation measures. Other, less influential, stressors included tidal barrages, artificial light at night, impingement in power-station cooling waters and thermal discharges, pile-driving noise pollution, invasive non-native species, electromagnetic fields, salmon mariculture, and tidal lagoons. Salmon fisheries exploitation was not regarded as an important stressor currently because effective exploitation rate controls have been implemented to substantially reduce fishing pressure. Future research priorities include addressing knowledge gaps on expanding stressor impacts from climate change, predation, renewable energy developments, and artificial light at night. Local management actions directed towards improving freshwater and estuarine habitats to maximise ecosystem resilience to stressors and minimise their cumulative impacts are recommended.
Article
In the present study we utilize tag recapture data to estimate year class abundance and spawning stock biomass of mackerel (Scomber scombrus L.) in the Northeast Atlantic for the period 1986–2008. On average 20,000 jigged mackerel have been tagged annually with internal steel tags in the spawning area west of Ireland and the British Isles, and the tags have been recaptured in commercial catches screened through metal detectors. The spawning stock biomass estimates derived from two different tag-based models were highly variable but were on average 2 and 2.3 times higher than the ICES official estimate. The official estimate is considered uncertain and most likely an underestimate of the actual biomass, due to unregistered mortality in the fisheries and lack of fishery-independent, age-disaggregated data. Hence, tag-based estimates could potentially improve the current assessment if included in the ICES stock assessment on a regular basis. These estimates also involve some uncertainty that needs consideration, especially related to variable tagging mortality, detector efficiency and migrations of the stock.
Article
Series of trials in which mackerel (Scomber scombrus L.) were confined in keepnets at different stocking densities are described. From simple confinement trials it was found that 50% of the fish died after 48 h at a stocking density of 30 fish m−3, equivalent to 6.5 kg m−3. Trials in which fish were held at stocking densities, and for a duration, comparable to those experienced in a “dried up” purse seine prior to “slipping”, showed that up to 90% of “slipped” fish died within 48 h of release. The primary cause of death was probably physical damage, particularly skin loss, caused by abrasion, although there is some evidence that mackerel have a healing process which can accommodate minor skin abrasions. A tagging trial showed a small but significant increase in mortality due to the tagging procedure.
Internal tagging experiments of mackerel in the Skagerrak and the 392 northeastern North Sea
  • J Hamre
Hamre, J., 1970. Internal tagging experiments of mackerel in the Skagerrak and the 392 northeastern North Sea. ICES CM 1970/H:25.